Respiration

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Notes

Respiration in Cells

**Respiration** is a chemical process that breaks down glucose to release energy; it is **enzyme-controlled**.
Respiration can be **aerobic** (with oxygen) or **anaerobic** (without oxygen).
Aerobic respiration releases much **more energy** per glucose molecule than anaerobic respiration.
Most aerobic respiration reactions occur in the **mitochondria**.
Energy released is used for: **muscle contraction**, **protein synthesis**, **cell division**, **growth**, **active transport**, **nerve impulses**, and **maintaining body temperature**.

Aerobic Respiration

Aerobic respiration requires oxygen and is the **complete breakdown** of glucose.
Word equation: **glucose + oxygen → carbon dioxide + water** (+ energy).
Balanced symbol equation: **C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O**.
This equation is the reverse of photosynthesis.

Anaerobic Respiration in Animals

Anaerobic respiration does **not use oxygen** and is an **incomplete breakdown** of glucose.
In animals (e.g., during vigorous exercise), glucose is converted to **lactic acid**.
Word equation: **glucose → lactic acid** (+ a small amount of energy).
Lactic acid build-up lowers pH in muscle cells, which can **denature enzymes**.
Lactic acid is transported to the **liver** where it is oxidised (using oxygen) back to carbon dioxide and water – this is **repaying the oxygen debt**.

Anaerobic Respiration in Yeast

In yeast, anaerobic respiration produces **ethanol** and **carbon dioxide**.
Word equation: **glucose → ethanol + carbon dioxide** (+ a small amount of energy).
Used in **bread making** (CO₂ makes dough rise) and **brewing** (ethanol in beer, CO₂ gives fizz).

Comparing Aerobic and Anaerobic Respiration

Aerobic: uses oxygen, complete breakdown, large energy release, products CO₂ and H₂O.
Anaerobic (animals): no oxygen, incomplete, small energy release, product lactic acid only.
Anaerobic (yeast): no oxygen, incomplete, small energy release, products ethanol and CO₂.
Anaerobic respiration in animals does **not** produce carbon dioxide.

Effect of Temperature on Respiration Rate

Respiration rate can be measured using **methylene blue** dye – it turns from blue to colourless as oxygen is used up.
**Independent variable**: temperature; **dependent variable**: time for colour change (rate of respiration).
Rate increases up to ~40 °C due to more kinetic energy and enzyme–substrate collisions.
Above 40 °C, rate decreases because **enzymes denature** (active site changes shape).
Control other variables: volume/concentration of dye, yeast suspension, glucose, and pH.

Animal cell showing mitochondria where aerobic respiration occurs.

Practice questions

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1.Glucose is broken down in the chemical processes of aerobic respiration. Which row of the table below shows the correct products of this breakdown?
Easy
- Aenergy, water, lactic acid, carbon dioxide
- Benergy, water, lactic acid, carbon dioxide
- Cenergy, water, lactic acid, carbon dioxide
- Denergy, water, lactic acid, carbon dioxide
2.Which of the following is the correct word equation for aerobic respiration in plants?
Easy
- Aglucose + oxygen → carbon dioxide + water
- Bcarbon dioxide + water → glucose + oxygen
- Cglucose + water → carbon dioxide + oxygen
- Dglucose + carbon dioxide → water + oxygen
3.Which of the following processes require, and are dependent on, the activity of enzymes? 1. aerobic respiration 2. glucose → lactic acid 3. osmosis 4. anaerobic respiration in yeast
Easy
- A1 and 2
- B1 and 3
- C1, 2 and 4
- DAll of them
4.A student investigated respiration in yeast. Every 30 minutes they replaced the water with the same volume but it was 10°C warmer and counted the bubbles given off in 2 minutes. What are the independent and dependent variables in this experiment?
Easy
- Aindependent: number of bubbles, dependent: time
- Bindependent: time, dependent: number of bubbles
- Cindependent: water temperature, dependent: number of bubbles
- Dindependent: water temperature, dependent: volume of water
5.Which of the following do humans require energy for? 1. Producing proteins 2. Cell division 3. Generation of nerve impulses 4. Maintaining body temperature
Easy
- A1 & 3
- B2, 3 & 4
- C4 only
- DAll of the above
6.Which row in the table below shows the correct products produced by anaerobic respiration in yeast and in animals?
Medium
- Aanimals: lactic acid (no), carbon dioxide (yes); yeast: lactic acid (no), carbon dioxide (no)
- Banimals: lactic acid (yes), carbon dioxide (no); yeast: lactic acid (no), carbon dioxide (yes)
- Canimals: lactic acid (no), carbon dioxide (yes); yeast: lactic acid (yes), carbon dioxide (no)
- Danimals: lactic acid (yes), carbon dioxide (yes); yeast: lactic acid (no), carbon dioxide (no)
7.During vigorous exercise, such as cycling uphill, lactic acid builds up in the muscles. How is this lactic acid removed during recovery?
Medium
- Aexcretion of lactic acid by the kidneys
- Banaerobic respiration of lactic acid in the muscles
- Caerobic respiration of lactic acid in the liver
- Dexcretion of lactic acid by the lungs
8.Why does anaerobic respiration of glucose release less energy in muscle cells than aerobic respiration? 1. Energy is lost in oxygen. 2. Energy remains trapped in the chemical bonds in the molecule lactic acid. 3. Energy remains trapped in the chemical bonds in the molecule ethanol. 4. Energy is lost in carbon dioxide.
Medium
- A1, 2 and 4
- B2 only
- C2 and 3
- D4 only

Slides

What is Respiration?

Respiration is a chemical process that breaks down glucose to release energy.
It is controlled by enzymes and takes place in all living cells.
Respiration can occur with oxygen (aerobic) or without oxygen (anaerobic).
Aerobic respiration releases much more energy per glucose molecule than anaerobic respiration.

Which of the following best describes respiration?

The process of breathing in and out
A chemical reaction that releases energy from glucose
The production of glucose from carbon dioxide and water
The breakdown of proteins in cells

Uses of Energy from Respiration

Energy released during respiration is used for:
Muscle contraction, protein synthesis, and cell division.
Active transport, growth, generation of nerve impulses.
Maintaining a constant internal body temperature.

Which of these processes require energy from respiration?

Muscle contraction
Diffusion of oxygen into cells
Protein synthesis
Maintaining body temperature

Aerobic Respiration

Aerobic respiration uses oxygen to completely break down glucose.
It releases a relatively large amount of energy.
The products are carbon dioxide and water.
Word equation: glucose + oxygen → carbon dioxide + water (+ energy)

What is the correct word equation for aerobic respiration?

glucose + oxygen → carbon dioxide + water
carbon dioxide + water → glucose + oxygen
glucose → lactic acid + energy
glucose → ethanol + carbon dioxide

Balanced Chemical Equation for Aerobic Respiration (Extended)

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy)
This is the reverse of the photosynthesis equation.
Make sure you can write and balance this equation for exams.

The balanced equation for aerobic respiration is: C₆H₁₂O₆ + ____ O₂ → ____ CO₂ + 6 H₂O

Anaerobic Respiration in Animals

Anaerobic respiration does not use oxygen.
It is the incomplete breakdown of glucose, releasing less energy.
In animals (e.g., muscle cells during vigorous exercise), it produces lactic acid.
Word equation: glucose → lactic acid (+ energy)

What is the product of anaerobic respiration in human muscle cells?

Ethanol and carbon dioxide
Lactic acid
Carbon dioxide and water
Oxygen and glucose

Anaerobic Respiration in Yeast

Yeast can respire anaerobically, producing ethanol and carbon dioxide.
Word equation: glucose → ethanol + carbon dioxide (+ energy)
This is used in bread-making (CO₂ makes dough rise) and brewing (ethanol in beer).

Which row correctly shows the products of anaerobic respiration in yeast?

lactic acid and carbon dioxide
ethanol and carbon dioxide
ethanol and water
lactic acid and ethanol

Oxygen Debt (Extended)

During vigorous exercise, lactic acid builds up in muscles, lowering pH.
After exercise, lactic acid is transported to the liver and oxidised using oxygen.
This requires extra oxygen, known as 'repaying the oxygen debt'.
That's why breathing and heart rate stay high after exercise.

How is lactic acid removed from the body after exercise?

It is excreted by the kidneys
It is oxidised in the liver using oxygen
It is converted back to glucose in muscles
It is exhaled through the lungs

Comparing Aerobic and Anaerobic Respiration

Aerobic: uses oxygen, complete breakdown, large energy release, products CO₂ and H₂O.
Anaerobic: no oxygen, incomplete breakdown, small energy release.
In animals: product is lactic acid. In yeast: products are ethanol and CO₂.

Which statements about aerobic and anaerobic respiration are true?

Aerobic respiration releases more energy than anaerobic.
Anaerobic respiration in animals produces carbon dioxide.
Yeast produces ethanol during anaerobic respiration.
Both types of respiration are controlled by enzymes.

Investigating Respiration Rate in Yeast

Methylene blue dye can be used to measure aerobic respiration rate in yeast.
The dye turns from blue to colourless as oxygen is used up.
Faster colour change = faster respiration rate.
Temperature affects rate: increases up to ~40°C, then decreases as enzymes denature.

In an experiment investigating the effect of temperature on yeast respiration, what is the independent variable?

Time for colour change
Temperature
Concentration of glucose
Volume of yeast suspension

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